1. Field of the Invention
The invention relates to a combustor for a gas turbine engine and more particularly to a combustor having a plurality of axially staged pre-mixed fuel/air inlets and a piloting flame of the diffusion type at its head end.
2. Description of the Prior Art
It has become increasingly important, because of the national energy conservation policies and also because of increasing fuel expense, to develop gas turbine engines having a relatively high thermal conversion efficiency.
It is a known principle of the gas turbine engine that an increase of thermal efficiency can be accomplished by increasing the turbine inlet temperatures and pressures. However, it is also recognized that increasing the turbine inlet temperature in turn increases the production of certain noxious exhaust pollutants. Of principal concern is the emission of oxides of nitrogen.
The sources of the nitrogen for forming the nitrogen oxides (particularly NO and NO.sub.2 and subsequently identified as NOx) is the nitrogen in the fuel and generally identified as fuel bound nitrogen and the nitrogen present in the combustion air. Reduction of fuel bound nitrogen generally requires a pre-treatment of the fuel to reduce the nitrogen content, which can be prohibitively expensive. Thus, to enable the high temperature gas turbines of the future to meet the proposed NOx emission standards it is necessary to minimize the NOx attributable to formation from nitrogen in the combustion air during the combustion process.
It is recognized that NOx formed from the combustion air is significantly influenced by the flame temperature and the residence time of the nitrogen at such temperature. In the present state of the art, diffusion flame type combustors of large gas turbine engines (i.e., wherein fuel is introduced into the combustion chamber through a fuel nozzle for atomization and mixture with air within the chamber just prior to combustion) the combustion of the fuel/air mixture produces adiabatic flame temperatures of from 3100.degree. F. to 4300.degree. F. (The flame temperature of both liquid and gaseous fossil fuels come within this temperature range.) Although the hot combustion gas products are mixed with air for quenching the temperature of the gas products to a lower temperature, the existence of such high temperatures at the diffusion flame front is sufficient to produce an unacceptable amount of NOx.
Further, as the relationship between the production of NOx and the temperature is generally an exponential relationship, any reduction in the flame temperature for the same residence time, significantly reduces NOx production. Further, since there exists a finite time increment necessary to complete the combustion process, which is on the order of a few milliseconds, NOx reduction through a decrease in the residence time is limited to the point where appreciable CO and unburned hydrocarbon levels appear in the exhaust. Insofar as most gas turbine combustion systems are concerned, residence times already hover around this minimum value, and thus the only remaining alternative to obtain significant reduction in NOx formation is to lower the combustion flame temperature.
Previous methods of lowering flame temperature are to inject steam or water into the flame or circulate a coolant in pipes to the flame front. However, each method has obvious inefficiencies and mechanical problems. Thus, a significant reduction in NOx production requires that the diffusion flame process of the present combustors, with its attendant high flame temperature NOx generation, be modified to develop a lower temperature combustion flame. U.S. Pat. No 3,973,390 and No. 3,973,395 are somewhat pertinent to this concept, however in each instance a vaporized fuel rich mixture is introduced into a combustion zone for mixture with air therein prior to burning as ignited by a pilot flame. And, at such high temperature conbustion, the speed of ignition exceeds the ability to mix such that fuel rich burning occurs, still resulting in an unacceptable level of thermally produced NOx.